Centrifugal compressors are well known in the art and are used in many applications to compress a gas from a lower pressure to a higher pressure. The gas at the lower pressure enters an inlet of the centrifugal compressor and is compressed to a higher pressure by being accelerated by a rotating impeller and then sent into a diffuser surrounding the impeller, in which additional pressure is recovered by decelerating the gas. The gas is discharged from the diffuser to a volute and from the volute to an outlet thereof at the higher pressure.
In a centrifugal compressor, a high pressure seal is provided on the back of the impeller or in other words on the side of the impeller opposite to the inlet. This seal is typically a labyrinth type of seal that has a smaller diameter than the outer diameter of the impeller as defined by the outer rim thereof. High pressure gas seeps through the outer rim during operation of the compressor and such seal prevents this high pressure gas from pressurizing the entire back of the impeller. A lower pressure gas from a location such as the compressor inlet or an even lower pressure source if available, is introduced into a cavity formed at the back of the compressor, between the motor shaft driving the impeller and the high pressure seal. Leakage from the cavity is prevented by a shaft seal on the motor shaft driving the impeller. The pressure within this cavity acts on the resulting inner annular area behind the impeller along with the high pressure acting in the outer annular area between the outer rim of the impeller and the high pressure seal. The sum of these two forces produce a force that is known as the back disk force on the impeller. Acting inwardly of the impeller, is an eye side force produced by the onrush of gas entering the impeller and acting on the surface of the impeller facing to the inlet. In the prior art, the high pressure seal is sized so that during design conditions the eye side force is in balance with the back disk force.
Compressors are designed to operate within an operating envelope that can be plotted in what is referred to as a compressor map of pressure ratio between outlet pressure and inlet pressure versus flow rate through the compressor. On such a plot, a peak or best efficiency operating line is plotted in which for a given flow rate and pressure ratio, the energy consumption of the compressor is at a minimum. The plot also has another dimension of specific speeds which cross the peak efficiency operating line. If the pressure ratio falls within the compressor for a given speed, a point is ultimately reached when the compressor goes into what is referred to as surge. A surge event is therefore, produced by flow rate through the compressor falling below a minimum flow required at a given speed of the impeller of the compressor that is necessary to maintain stable operation. From a viewpoint taken from the impeller, a surge event has two phases. In a first phase there is a sudden loss of discharge pressure within the impeller flow passages arising from aerodynamic instability in the impeller. This results in a decrease in the eye side pressure. However, the back disk force, remains high due to fluidic system inertial effects. This results in a large axial force on the impeller driving the impeller towards the inlet. The second phase of the surge event is produced by the high pressure back disk pressure bleeding down to lower eye side pressure, resulting in the eye side force overcoming the weakened back disk force. This produces a large axial force driving the impeller away from the inlet. The frequency at which these forces are developed is quite high resulting in destruction of the compressor starting with an overload of the thrust bearing.
The forces developed during a surge event can be computed or measured and the thrust bearings in the compressor can be designed with an overload margin to absorb these forces and prevent damage to the compressor. However, there has been the increasing use of what are referred to as oil-free bearings in centrifugal compressors. These oil free bearings are either electromagnetic bearings or aerodynamic bearings. In electromagnetic bearings, the motor shaft is suspended in both radial and axial directions by means of a magnetic force. However, such oil-free bearings when applied to the thrust bearing are less able to adsorb the large axial forces that occur during a surge event than conventional bearings that are lubricated with oil. Thus, the overload margins that can be provided by oil-free bearings are less than those that can be provided by conventional bearings. As such, during a surge event, the thrust bearing will be unable to absorb the forces and back-up-, bearings or bushings will be contacted to adsorb the force. The problem with this is that back-up bearings or bushings can only be subjected to a predefined small number of usages during these surge events. After this predetermined number, the compressor will have to be taken off-line for maintenance to replace the back-up bearings or bushings.
The problems outlined above with respect to oil-free bearings have therefore, limited their use in large compressor applications such as air separation plants in which such bearings would otherwise have the advantage of not contaminating the feed to the plant with oil, having much lower frictional losses, and will require less maintenance than conventional oil lubricated bearings. In an application such as an air separation plant, if a compressor is taken off-line, the plant will not function. Unplanned outages in any large installation will in any case result in financial hardship to the plant operator.
As will be discussed, unlike the prior art, the present invention provides a method and apparatus in which the impeller is preloaded with a biasing force that is larger than that required to simply balance impeller eye side and back disk forces during normal operation of the compressor. This preload force is used to increase an overload margin of the bearings that could be exceeded during a surge event and thus, is particularly applicable to a centrifugal compressor using oil-free bearings.